Electrochemical energy storage systems, such as batteries, supercapacitors and the like, have been widely proposed for large-scale energy storage applications. Various battery designs, including flow batteries, have been considered for this purpose. Compared to other types of electrochemical energy storage systems, flow batteries can be advantageous, particularly for large-scale applications, due to their ability to decouple the parameters of power density and energy density from one another.
Flow batteries generally include negative and positive active materials in corresponding electrolyte solutions, which are flowed separately across opposing faces of a membrane or separator in an electrochemical cell containing negative and positive electrodes. The terms “membrane” and “separator” are used synonymously herein. The flow battery is charged or discharged through electrochemical reactions of the active materials that occur inside the two half-cells. As used herein, the terms “active material,” “electroactive material,” “redox-active material” or variants thereof synonymously refer to materials that undergo a change in oxidation state during operation of a flow battery or like electrochemical energy storage system (i.e., during charging or discharging).
Although flow batteries hold significant promise for large-scale energy storage applications, they have historically been plagued by sub-optimal energy storage performance (e.g., round trip energy efficiency) and limited cycle life, among other factors. Certain factors leading to sub-optimal performance are discussed hereinafter. Despite significant investigational efforts, no commercially viable flow battery technologies have yet been developed.
Balanced oxidation and reduction of the active materials in a flow battery are desirable electrochemical reactions, since they contribute to the battery's proper operation during charging and discharging cycles. Such reactions may be referred to herein as “productive reactions.”
In addition to desirable productive reactions, undesirable parasitic reactions can also occur within one or both half-cells of flow batteries and related electrochemical systems. As used herein, the term “parasitic reaction” refers to any side electrochemical reaction that takes place in conjunction with productive reactions. Parasitic reactions can often involve a component of an electrolyte solution that is not the active material. Electrochemical reactions of an active material that render the active material unable to undergo reversible oxidation and reduction can also be considered parasitic in nature. Parasitic reactions that can commonly occur in electrochemical cells containing an aqueous electrolyte solution are evolution of hydrogen and/or oxidation by oxygen. Hydrogen evolution, for example, can at least partially discharge the negative electrolyte solution of an electrochemical cell. Related parasitic reactions can also occur in non-aqueous electrolyte solutions.
Discharge associated with parasitic reactions can decrease the operating efficiency and other performance parameters of a flow battery. In addition, parasitic reactions can change the pH of an electrolyte solution, which can destabilize the active material therein in some cases. In the case of a parasitic reaction that occurs preferentially in one half-cell over the other, an imbalance in state of charge can result between the negative and positive electrolyte solutions. The term “state of charge” (SOC) is a well understood electrochemical energy storage term that refers herein to the relative amounts of reduced and oxidized species at an electrode within a given half-cell of an electrochemical system. Charge imbalance between the electrolyte solutions of a flow battery can lead to mass transport limitations at one or both of the electrodes, thereby lowering the round-trip operating efficiency. Since the charge imbalance can be additive with each completed charge and discharge cycle, increasingly diminished performance of a flow battery can result due to parasitic reactions.
Charge rebalancing of one or both electrolyte solutions can be conducted to combat the effects of parasitic reactions. Although various charge rebalancing techniques are available, they can be costly and time-consuming to implement. Determining the true concentration of oxidized and reduced active material species in an electrolyte solution can oftentimes itself be difficult, thereby adding a further difficulty to the charge rebalancing process. While charge rebalancing of an electrolyte solution can often be accomplished given sufficient diligence, the accompanying pH changes can frequently be much more difficult to address.
In view of the foregoing, flow batteries and other electrochemical systems configured to decrease the incidence of parasitic reactions and other performance-reducing factors would be highly desirable in the art. The present disclosure satisfies the foregoing needs and provides related advantages as well.